1. Field of Invention
The present invention relates to systems and methods for efficiently applying power to a plurality of loads, including controlling switching of circuits between a primary power source and at least one secondary power source.
2. Discussion of Related Art
Transfer switches are often used in association with portable or stand-alone electrical generators in order to switch a source of electricity for a load between a utility electricity supply and the back-up generator.
For example, a private residence may normally receive its electrical power from a utility company. For various reasons, however (location in a region prone to severe weather, unreliable utility power, etc.), a homeowner may desire a back-up source of electrical power so that electrical devices and appliances in the residence can be powered during periods in which utility power is unavailable. The back-up generator may typically be a gasoline, diesel, propane or natural gas internal combustion engine-powered electrical generator, capable of generating an alternating voltage. The back-up generator may be installed in or near the residence and arranged to be connected to one or more of the electrical circuits in the residence in order to provide the desired back-up power.
It may often be undesirable and often illegal or a code violation to leave the back-up generator permanently connected, in parallel with the utility power, to the residential electrical circuits. Thus, conventional residential transfer switches, typically including a manually actuated switch, are used to connect and disconnect the back-up generator. For example, when the switch is thrown, the connection between the residential circuit(s) and whichever power source (i.e., utility or generator) is at the time actually connected to the residential circuit, is broken, before the connection is made between the residential circuit, and the power source which is being substituted in.
Conventional manual transfer switches are capable of providing the simple function of serving to safely accomplish the substitution of power sources. However, such manual switches typically require the presence of an operator in order to accomplish the transfer. This may be undesirable, in that some appliances (e.g., refrigerator or freezer, sump pump, etc.) should not go without power for extended periods of time and an operator may not be present at the time of a power outage to throw the switch. In addition, manual transfer switches require the circuits to be manually switched to the generator, and as such have no means of load control or overload protection other than fuses or circuit breakers. Manual transfer switches also have no automatic means of controlling non-critical loads.
Also available are conventional automatic transfer switches that are capable of actuating automatically, upon sensing a sustained interruption of utility-supplied electrical power. Traditionally, a generator used with the switch is selected to match the load, however, an overload may cause the generator to stall or a circuit breaker to trip, or both. Automatic transfer switches, if they do come with load-shedding features, generally require a complicated installation and configuration process.
One example of an automatic transfer switch and a method of load control for an automatic transfer switch that sheds and reconnects loads based upon a “preselected prioritized hierarchy” is described in U.S. Pat. No. 6,172,432 B1 to Schnackenberg et al., entitled “Automatic Transfer Switch,” which is herein incorporated by reference in its entirety. The system of the '432 patent provides the benefit of preventing generator overload while making sure high priority loads remain powered. A disadvantage of the system of the '432 patent, however, is that the use of all lower-priority loads may be prevented if a high-priority load requires power.
Generators may be used without transfer switches, however, this may be dangerous due to the possibility of backfeed into the utility power or shock hazard. As discussed above, manual transfer switches do not provide any load control and are easy to overload. Overloads can happen during initial inrush or during steady state operation. When a generator (or other backup source such as a UPS) is overloaded during a blackout, a manual restart or reset may be required. Thus, power to critical loads may be turned off inadvertently, but permanently, through, for example, a non-critical load causing an overload.
Conventional generators also do not provide uninterrupted power because they typically require ten or more seconds to start. UPS's do provide this capability, but they are typically designed as point backup and can be difficult to hardwire into building wiring. Using both generators (for long term backup) and UPS's (for short term, uninterrupted power) is a good solution but is often difficult and expensive to design and install for the average homeowner or small business.
Existing methods and systems also do not account for the possibility of a third source (i.e. a UPS or a second generator), and are often expensive while providing only limited functionality. There is thus a need for an automatic transfer switch that is easily and safely connectable, cost effective, and provides wide range of adaptability and functionality.